This invention relates to an olefinic block copolymer having excellent well-balanced flexibility, strength and processability and also to a partially crosslinked product derived from this copolymer and having excellent heat resistance and set property.
In recent years, soft materials having excellent heat resistance (particularly form retention against external forces at high temperatures) and strength have been developed for automobile parts, domestic electrical article parts, coating materials of electrical cables, pipes, etc. These materials have physical properties which are comparable to or approximate those of vulcanized rubbers and yet also possess moldability which is deficient in vulcanized rubbers. For this reason, these materials are now found to have created one field of materials as thermoplastic elastomers. Particularly polyolefinic thermoplastic elastomers, differing from other polystyrene type or polyester type elastomers which are obtained by block copolymerization of hard segments and soft segments, have been investigated primarily with respect to kneaded products of polyolefinic resins as hard segments (e.g., polypropylene, polyethylene, poly(ethylene-vinyl acetate) and others) and polyolefinic rubbers as soft segments and also sometimes with respect to products of which the rubber portions are partially crosslinked during kneading. Practical developments have been made mainly by the use of a system of poly(ethylene-propylene) type rubber/polypropylene type resin, as disclosed in Japanese Patent Laid-open Publication No. 53938/74. However, these materials are accompanied by the problem in their manufacturing process of requiring an additional step of fusion kneading of both components, whereby the cost is increased to a great extent to hamper expansion of the demand. In addition, there is also the problem of quality, because molecular designs of both components have not heretofore been sufficiently investigated with respect to the characteristics to be possessed by the thermoplastic elastomers. In particular, these materials of the prior art were not satisfactory with respect to the balance between flexibility, strength, and moldability, and also with respect to heat resistance and the balance between reduced permanent-set tendency and moldability in the case of crosslinked thermoplastic elastomers.